Various solutions which become loaded with silver as they are used are employed in the processing of photographic negatives and prints, X-rays, lithographs, and other items of a comparable character. Exemplary of such solutions are fixers; bleaches; wash water; the bleaches, fixers, and stablizers employed in waterless developing processes; and the tailings from electrolytic cells.
It is often desirable to recover the silver from the spent solution. This may be desirable or necessary for pollution control purposes or because the concentration of silver in the spent solution is high enough to make the recovery of the silver economically desirable.
Heretofore, units designed to recover silver from solutions of the character described above have typically operated on an ion exchange principle. The spent, silver-containing solution percolates through a gravity flow cartridge containing a spool of continuously wound steel wool under reaction conditions which cause the silver ions to undergo exchange reactions with ferrous ions of the steel wool matrix. This reduces the silver to an elemental form, the metal plating out on the steel wool and/or accumulating on the bottom of the gravity flow cartridge.
One disadvantage of heretofore available silver recovery units of the character just described is that various insoluble compounds tend to precipitate out of the solution being processed as the exchange reactions proceed, especially if the volume of solution being processed is low and the solution is therefore flowing only slowly through the recovery unit. These precipitates tend to clog the pores in the recovery medium, and this leads to channeling by virtue of the solution being forced into a restricted number of paths and dissolving holes through the reaction medium. Channeling is undesirable because it decreases the recovery medium surface area which is available to the spent solution, thereby decreasing the efficiency of the recovery unit. At the same time, the solution being processed flows faster through the reaction medium, decreasing the contact time and thereby even further decreasing the efficiency of the recovery process.
Also, plugging of the interstices in the recovery medium of a gravity flow recovery unit further reduces the efficiency of the unit by reducing the surface area of the reaction medium available to the solution being processed and by impeding the flow of the solution therethrough, apart from the adverse effect on efficiency which any channeling might produce.
Yet another drawback of conventional cartridges of the character described above is that they are very much subject to leakage which is undesirable because of the resulting mess and because the leaking solution may be of a corrosive or other harmful character.
Also, conventional cartridges have the disadvantage that they can not be pressurized to eliminate the problems discussed above and associated with gravity flow because the cartridge or container housing the reaction medium will burst.
Another decided disadvantage of typical silver recovery systems of the conventional, cartridge type is the lack of adequate control over the rate at which the solution being treated flows to the ion exchange cartridge. This lack of adequate flow rate control also contributes to pore clogging and the resultant drop in efficiency in conventional silver recovery units of the ion exchange type.
It is also a disadvantage of those units that the continuous filament reaction media they employ is relatively inefficient as far as the reactions involved in the ion exchange recovery of silver are concerned.
Still other processes and devices for recovering and precipitating metals such as those with which I am concerned are disclosed in U.S. Pat. Nos. 1,349,086 issued Aug. 10, 1920, to Murphy for METAL PRECIPITATOR AND SEPARATOR; 2,905,323 issued Sept. 22, 1959, to Megesi for APPARATUS FOR RECOVERY OF SILVER FROM SPENT PHOTOGRAPHIC SOLUTIONS; 3,043,432 issued July 10, 1962, to Megesi for APPARATUS FOR RECOVERY OF SILVER FROM SPENT PHOTOGRAPHIC SOLUTIONS; 3,630,505 issued Dec. 28, 1971, to MacKay for SILVER RECOVERY; 3,692,291 issued Sept. 19, 1972, to MacKay for SILVER RECOVERY; 3,744,995 issued July 10, 1973, to MacKay for SILVER RECOVERY; and 3,840,217 issued Oct. 8, 1974, to MacKay for SILVER RECOVERY. Without exception, the foregoing patents are concerned with processes and systems of the gravity flow type although Megesi '432 does make the statement that "auxiliary pumping means" were used in some undisclosed manner "in commercial installations . . . to force the fluid through the apparatus." Therefore, the just-identified, heretofore patented systems would have those disadvantages of gravity flow systems discussed above.
Furthermore, the patented systems employ as reaction media such undesirable materials as steel wool (Megesi '323 and Megesi '432), which was discussed above; turnings and chips, which have the disadvantages that flow through the reaction mass is apt to be sluggish and that the surface area accessible to the solution being processed is apt to be less than optimal; and screening, which is also undesirable because it, too, is relatively inefficient.
In addition, the foregoing patented devices do not appear to be of a construction which would resist leakage or bursting if operated under pressure. Consequently, the patented devices could not be employed in the manner discussed above and found by me to be advantageous.
Nor do the patented systems have control systems which like mine, would be capable of maintaining a constant rate-of-flow of the liquid being treated and of thereby keeping precipitates from plugging the interstices in the reaction medium and/or the reaction vessel inlet or outlet.